A few months ago, I knew almost nothing about carburettors, but through necessity, I have now become well acquainted with this little Zenith Carb. Basically, the purpose of the carburettor is to inject the correct amount of petrol into the engine’s air intake stream so that the fuel to air ratio is correct across the engine’s full range of operation
The amount of air flow through the carburettor and into the engine is controlled by the throttle butterfly vane valve.. This air intake air passes through a venturi (also called a choke tube ) within the carburettor which creates a proportional vacuum allowing the correct metered amount of fuel to enter the air stream.
This carburettor may be fitted with either a fixed or variable main jet. From this chart, you can see that the required fixed main jet for this JAP 5 engine should be 49 (this has a hole diameter of 0.49mm )
The alternate to using a fixed jet of the correct size is to use a variable jet, which consists of an adjustable needle valve in use with a larger sized fixed jet……..by fine adjustment of the needle valve , you should be able to get the desired result.
The carb can either be fitted with a fixed jet…….components 20,21,22,23 or a variable jet , components 24,24,26 and 27
When I started out using my Anzani Iron Horse, it started easy from cold , ran a little rough but operated OK. However, I could not start it when hot. In order to restart it , I had to let it completely cool down. One evening I just gave up on trying to restart it and left it in the field overnight. When I returned the next morning, it started straight away and I moved it up the field towards the road, but then when It stopped, I couldn’t restart it again. It was time to remove the carb and investigate.
This was my carb as I first removed it. It was fitted with a variable main jet. The knurled cap on the needle valve has an indicator attached to it so that it is possible to measure / estimate its rotation. This is adjusted by first turning in fully (but gently to avoid damaging the needle valve) and then backing out. (pehhaps 1 full turn back out initially )
I opened the carb by separating its top and bottom. ( 5 bolts ) The joint gasket was made from a biscuit packet cardboard !
My carb did not have air jet part 45 fitted. I removed some dirt particles from the chamber, cleaned up the filter gauze in the fuel inlet banjo ( part 11 ) and blew compressed air through all its passages and reassembled.
The float was in good condition and the inlet needle valve was working OK
To ensure that the top and bottom of the carb were mating correctly together, I removed any high spots on the joint by lapping on a sheet of wet emery placed on a sheet of glass. I also fitted a new joint gasket made from a sheet of gasket paper.
Meanwhile, I attended a couple of ploughing matches but on each occasion I didnt even get the opening complete , before I spent the rest of the day cranking and cranking , but no joy. Had to get towed by a fellow competitor from the ploughing area to the head of the field to load onto trailer. I then decided to send the complete carb away to be refurbished and get the correct fixed jet installed. Paul Child at Meetens was very helpful in getting the carb turned around.
So when I got the carb back and fitted , I was`ready for the next ploughing match. It was certainly starting easy from cold and was idling better than ever before. However I was concerned still with the excess smoke when I revved up the engine. So unfortunately I was stopped again before I completed the opening. It was putting out a lot of smoke and had no power. Although the carb had just been refurbished, we suspected that the new main jet fitted could not be correct. I opened the carb to discover that the jet size fitted was an 85 ! ( I was expecting a size 49 to be fitted ) So when my fellow ploughers were finished their plots , we tool the bottom off one of their carbs and fitted it to mine ………….Result at last………………..it ran perfectly and so I got to plough no problem. So now I knew it was just a matter of getting the correct fixed jet installed.
While i was managing to get by with excess fuel supply when the engine was cold, once the engine heated up , the excess fuel was giving such a rich mixture that it was simply smothering the engine.
The smallest jet Meetens had available was a size 62 , so i received a couple of these and a variable nozzle assembly from them. I fitted the size 62 fixed jet , and although it was an improvement it was still not good enough. I had planned on switching out to the new variable jet but at my most recent ploughing event, Richard Duffy turns up with some fixed nozzles he made on his lathe and milling machine. I installed one of these and haven’t looked back since ! Many thanks to Richard for his help on this . Creating one of these small precision parts to such precise specification truly demonstrates great skill.